Large satellite communication systems are expensive to build and place in orbit. Because of the expense, it is not typically feasible to supplement present day satellite communication systems with other satellite communication systems if the present day systems have excess useable capacity.
A "satellite" is defined herein to mean a man-made object or vehicle intended to orbit Earth and includes both geostationary and orbiting satellites and/or combinations thereof. A "constellation" is defined herein to mean an ensemble of satellites arranged in orbits for providing specified coverage (for example, radio communication, remote sensing, etc.) of a portion or all portions of the celestial body. A constellation typically includes multiple satellites which may be arranged in multiple rings (planes) and may have equal numbers of satellites in each plane, although this is not essential.
A direct communication link between two satellites is referred to herein as a "crosslink". A crosslink is maintained where both the source and the destination or intermediate satellite's communication antennas are pointed toward each other and data communication is occurring. In a satellite communication system where satellites travel in the same direction, only small adjustments to a communication antenna's position are necessary to maintain a crosslink.
During each orbit, a non-geosynchronous satellite will travel in both an ascending (i.e., northbound) and a descending (i.e., southbound) direction. Where multiple, parallel orbital planes exist, satellites in a first orbital plane may travel in the same direction as satellites in an adjacent orbital plane, or the satellites in the first orbital plane may travel in the opposite direction (e.g., satellites in a first orbital plane are ascending and satellites in an adjacent orbital plane are descending). The space between these "counter-rotating" orbital planes is referred to herein as a "seam".
Typical satellite communication systems do not provide a method for maintaining a crosslink between satellites in adjacent orbital planes that are traveling in opposite directions. For this reason, crosslinks are not fully utilized. Satellites positioned on the "edge" of the constellation do not use all of the crosslink capability they have on-board. Typically, there is a seam on each side of the celestial body about which the satellites are orbiting.
In satellite communication systems generally, resources are managed by allocating channels to communication units as they become available within a given service area. If subscribers attempt to access the system while all resources are busy, they are generally denied access and are instructed to re-attempt accessing the system at a later time. In cellular communication systems, resources are expanded by partitioning service areas into cells and allowing reuse of communication resources. This reuse permits a greater number of users to access a satellite communication system.
In space-based cellular communication systems, however, additional potential users may not be located on the surface of the earth. Space-based communication systems must provide capabilities and mechanisms which can be used to provide communication services to additional users located in space.
Space based resources must be managed to allow additional users to gain access to the system. In order to increase services, communication links must be established between the additional resource and satellites in the existing constellation. If resources required for an additional link are unavailable, then the communication link will not be established by the additional resource. Allowing space based resources to be under utilized is unacceptable in a commercially viable communication system.
Accordingly, there is a significant need for apparatus and methods that allow acquisition of communication services by resources which are located in a space environment and provide a reliable communication link between existing satellite communication systems and new resources.
There is also a significant need for apparatus and methods which enable the processing and/or data carrying capacity within a particular satellite communication system to be increased relative to new demands and in conjunction with state-of-the-art technology. In addition, there is a significant need for apparatus and methods for increasing the utilization of the on-board resources of orbiting satellites without modifying the satellites.